1. Field of the Invention
The present invention relates to lasers and, in particular, to a laser having an improved laser rod cooling arrangement.
2. Description of the Prior Art
A solid state laser rod, before it will emit electromagnetic radiation, must be excited by an outside source of energy. This source of energy emits visible or near visible radiation itself which is converted by the laser rod into a laser beam. Solid state laser rods generally are relatively inefficient and, therefore, convert only a relatively small portion of the energy absorbed by them into laser radiation, with much of the remaining energy being converted into heat. Because the efficiency of most solid state laser rods decreases with an increase in temperature, the additional heat generated by the laser's inefficient utilization of the outside source of energy exacerbates the inefficient operation of the laser. Thus, in order to counteract these heating and related inefficient operating problems, heat must be removed. A number of techniques have been utilized in the prior art to dissipate the generated heat. For example, if the heating is more than about 3 watts (about 35 watts of electrical power to the lamp), the rod can be cooled by flowing a fluid, such as a liquid or pressurized gas, over the rod. The fluid is recycled through a heat exchanger. Such a dynamic cooling system has various drawbacks. In the case of pressurized gas, a major problem is the high cost of sealing the circulating coolant gas while still permitting transmission of the light from the lamp. For liquid systems, the concern is the ultimate deterioration of the fluid upon exposure to the lamp, or pump light.
Because of these problems, other efforts relate to cooling the rod passively by thermal conduction. This technique is exemplified in U.S. Pat. No. 4,210,389 to Burkhart et al, in which the rod is cooled by depositing a reflective metallic layer on one side of the rod which is soldered to a heat sink. This technique has been found to grossly distort the optical quality of the rod for at least two major reasons. First, because the rod is rigidly held in a thermally conductive support and the two have different coefficients of thermal expansion, stresses are developed during pumping and cooling; the rod grows at a rate different from its support. This unequal expansion induces stress birefringence. Second, the rod is supported on the support along its length on only a portion of its periphery, which creates a thermal gradient within the rod and a consequent nonuniform cooling. The result is an introduction of optical aberrations in the rod.
U.S. Pat. No. 4,181,900 to Tajnai et al also describes a conductively cooled pump cavity, in which the laser rod is strapped to a heat sink. The Tajnai et al system does not provide a satisfactory technique for cooling the laser rod since there is no thermal contact around the entire periphery of the rod. The result, like U.S. Pat. No. 4,210,389, is uneven rod cooling with induced thermal stresses. In addition, the straps themselves mechanically stress the rod.
Thus, what is desired is an improved technique for conductively cooling laser rods, which avoids the above stated disadvantages.